1. Field of the Invention
The present invention relates generally to techniques for generating limit-cycle oscillations (LCOs) in switch-mode power supplies (SMPSs), and more specifically, to a switch-mode power supply in which the transfer function of a digital element in the control feedback loop of the SMPS is temporarily altered in order to generate an LCO.
2. Background of the Invention
Switching power converters, referred to as switch-mode power supplies (SMPSs) are currently in widespread use for applications such as systems power supplies, AC power inverters, as well as localized power supplies, also known as point-of-load (PoL) supplies, such as voltage regulator modules (VRMs) for microprocessors. In a SMPS, one or more magnetic storage elements such as inductors or transformers are energized and interrupted by a switching circuit and the stored energy is typically periodically transferred to one or more capacitive storage elements. The output voltage or output current (or an analog of the output voltage/current) of the SMPS is sensed by a sensing circuit and used to control the switching circuit so that voltage or current regulation is provided over a variety of input voltage, output load and temperature variation ranges.
A compensation circuit or “compensator” is provided in the feedback and/or feed-forward paths of the converter between the sensing circuit and the switching circuit and sets the control response of SMPS to the sensed output voltage and/or current. The compensator modifies the closed-loop response of the converter response to ensure that the converter is stable, i.e., the output is well-behaved, and to ensure desired operating conditions. The crossover bandwidth is the bandwidth at which the converter loop gain becomes unity, and is a function of the reactance and resistance of the above-mentioned inductive and capacitive storage element(s), as well as the open loop gain of the converter circuits and the compensator. The crossover bandwidth is set to a frequency low enough that the phase shift around the converter loop is less than 180 degrees by a phase margin.
Since the reactance and resistance values of the capacitors and inductors used in SMPS can vary widely both from device-to-device and over temperature and device aging, a very conservative approach to compensation must typically be taken. Device-to-device variations can be compensated-for by production tuning, but at considerable cost and potentially high rejection rates if a conservative design is not chosen. Such conservative designs typically require capacitors having at least 40% greater capacitance than would be necessary for an optimally-tuned SMPS. The capacitors are typically the most expensive components of the SMPS and also one of the largest space and weight consumers, particularly for a high-frequency SMPS, in which the transformers and/or inductors can be made very small.
Even if a particular set of storage element parameters is known for an off-the-shelf SMPS design, the connected load, which also might be partially or fully capacitive, will change the characteristics of SMPS operation so that an ideal response is not possible for all applications. For example, when an SMPS is connected to digital equipment, the power supply distribution buses typically have large amounts of capacitance provided for decoupling and local energy storage to reduce the amplitude of transient voltage due to digital switching. The amount of capacitance will vary from application to application and the esr of the external capacitance and for some capacitor types (e.g., aluminum electrolytic capacitors) the capacitance itself will vary widely with operating temperature.
The design of such an “ideal” converter is further exacerbated for manufacturers of controller integrated circuits (ICs) intended for use in off-the-shelf SMPSs or use by other manufacturers in on-board SMPS designs that form part of a larger sub-system. The controller ICs must be able to implement SMPS compensators not only in varying applications, but for SMPS designs with wide ranges of storage element reactances and resistances.
The above-incorporated Parent U.S. Patent Application, as well as the other above-referenced related U.S. Patent Applications disclose and claim particular techniques for extracting component parameters of a particular SMPS and determining compensation for the particular SMPS without disrupting power supply characteristics at the input and output of the power supply. However, improvements are desirable to any technique that can provide information about SMPS component parameters and determine compensation responses that will yield desired operation, while avoiding disruption of the output of the SMPS and the power supply input source.
Therefore, it would be desirable to provide an improved method and system for determining the characteristic response of an SMPS. It would further be desirable to provide such a method and system that introduces little or no interference with the SMPS output and line input.